The present invention relates to a method of determining the starting ability of a starter battery of an internal combustion engine and a corresponding arrangement.
In a motor vehicle (or another system supplied with power primarily from an internal combustion engine), the functioning of the starter battery of the internal combustion engine is one of the most important prerequisites for ensuring good working order. An internal combustion engine is usually started by an electric starter which obtains electric power for the starting operation from a starter battery. The starter must briefly apply a torque capable of turning over the internal combustion engine at a certain minimum rpm. To do so, a sufficiently high current must be flowing through the starter, i.e., a sufficiently high voltage must be applied to it. If the starter battery is very cold, very old or partially discharged, the internal resistance of the battery may become so high that insufficient current and voltage are available to ensure starting. Such a problem is described in German Patent No. 197 05 634, for example.
For example, if the no-load voltage and the internal resistance of a starter battery are known, the starting ability of the starter battery for a given starting current and temperature can be checked. To this end, the terminal voltage can be calculated from known parameters and compared with the minimum voltage required to achieve the starting rpm. This is important, for example, to guarantee starting ability in start-stop systems in that the engine is started well in advance before reaching the starting limit in order to thereby recharge the battery through the generator.
Since the internal resistance of the starter battery also depends on the age of the battery in addition to its charge status and temperature, it must be determined again repeatedly at regular intervals. To obtain only the ohmic resistance of the electrodes and the electrolyte, which is crucial for the starting operation, one method is to calculate the internal resistance from the voltage dip and the maximum current in starting. However, the large current measurement range required for this purpose of up to 1 kA or more and the high current gradients of up to 1 kA/msec that occur are problematical in this regard. This makes high demands on the current sensors in a battery management system and under some circumstances results in the need for an additional sensor for this high current measurement range.
An object of the present invention is to find the simplest possible method of determining the starting ability of a starter battery without having to determine a starting current of the battery.
According to the present invention, it is now possible to determine the starting ability of a starter battery of an internal combustion engine without determining or calculating the starting current of the battery. Therefore, in the case of battery management systems which sense the battery current for determining the charge status, the current measurement range is greatly limited in comparison with traditional options, because it is possible according to the present invention to omit the determination of the charge drawn in starting, which is usually low.
The present invention utilizes, instead of the internal resistance and the starting current of the battery, the average voltage drop in the starter battery during starting to determine the starting ability. In this way, it is possible to eliminate complicated sensors for measuring the starting current.
According to a preferred embodiment of the method according to the present invention, the average voltage drop calculated according to the present invention is compared with a threshold value with the help of an equation of the form
xe2x80x83U0(soc, TBatt)xe2x88x92xcex94UBatt, mitt(soc, TBatt)=UBatt,min 
where soc is the charge status of the starter battery, TBatt is the temperature of the battery, U0 is the open-circuit voltage of the battery, xcex94UBatt, mitt is the average voltage drop in the battery during a starting phase and UBatt, min is the minimum voltage that can be used as a threshold value to achieve the starting rpm of the starter battery. It is usually possible to assume that minimum voltage UBatt min is known, charge status soc being supplied by the battery management system. On the basis of the known equation, it is also possible as an alternative to determine the minimum charge status for a successful start at a given starting temperature by solving the equation for soc.
According to another preferred embodiment of the method according to the present invention, the average voltage drop is calculated according to an equation
xcex94UBatt, mitt(soc, TBatt)xe2x88x921/n*xcexa3(U0(soc, TBatt)xe2x88x92UBatt, i)i=1 . . .n 
at times i=1 . . . n during the turn-over phase of the starter motor. This yields an averaging, which permits the starting ability of a starter battery to be reliably estimated.
Open-circuit voltage U0 of the battery at a known charge status is expediently determined according to an equation of the form
(U0(soc, TBatt)=6*(1.88+0,247*soc+(TBATTxe2x88x9225)*0,23exe2x88x923) 
at a known charge status and a known battery temperature and/or from the terminal voltage of the unloaded battery immediately before starting the internal combustion engine.
According to another preferred embodiment of the method according to the present invention, an engine characteristics map for the average voltage drop values is determined and stored as a function of the battery charge states and battery temperatures on the basis of average voltage drop values xcex94UBatt, mitt thus determined. Such an engine characteristics map makes is possible to determine the actual voltage drop values on the basis of the measurement or determination of the battery charge status and battery temperature through appropriate interpolation and extrapolation without having to perform specific calculations using the above equations.